Current standards for video compression permit image sequences to be compressed for transmission at relatively low bit rates, while providing flexible control of compression parameters in order to suit different networks and systems with different bit rates. Probably the most advanced video compression standard that is currently in broad use is ITU-T Recommendation H.264, “Advanced Video Coding for Generic Audiovisual Services” (International Telecommunication Union, March, 2005), which is incorporated herein by reference. This standard uses inter-picture prediction to encode image frames using previous frames in the sequence as references, as well as both lossless and lossy techniques for compressing image data within a frame.
A video encoder may employ rate control as a way to regulate varying compression characteristics of coded image frames in a video sequence, so as to produce an output bitstream at a given target bit rate. For H.264 encoders, for example, the degree of quantization of compressed image coefficients may be varied in order to achieve the desired output bit rate. Li et al. describe an adaptive rate control scheme that may be used for H.264 in “Adaptive Rate Control with HRD Consideration,” published as document JVT-H014, Joint Video Team (JVT) of ISO/IEC MPEG & ITU-T VCEG (ISO/IEC JTC1/SC29/WG11 and ITU-T SG16 Q.6), 8th Meeting (Geneva, May 20-26, 2003), which is incorporated herein by reference.
PCT International Publication WO 2006/085301, whose disclosure is incorporated herein by reference, describes a method for image adaptation based on a target size requirement. This publication points out that in some applications, it is desirable to adapt an image so as to comply with certain target characteristics, such as target file size and/or resolution. For example, some communication systems send images to be displayed by mobile communication terminals. The mobile terminals often have limited processing, memory and/or display capabilities. In such applications it is often desirable to adapt the images sent to a particular terminal in order to match the terminal capabilities, a process commonly referred to as media adaptation. Target file size limitations may also be imposed by the system to reduce traffic load. The publication describes a method by which an input image is modified by optimally determining at least one of the resolution and the quality of the input image so as to produce a compressed output image meeting the target size requirement.